1. Field of the Invention
This invention relates to controlling the bypass flow of a displacement pump. The invention provides for controlling the bypass flow in response to external load and/or flow requirements in a load responsive system. The invention also provides an internal response to load in a non-external load responsive system. The invention is a multi-purpose pressure control valve with low unload features.
My application Ser. No. 08/121,275, now U.S. Pat. No. 5,368,061, relates to pumps and the disclosures thereof are incorporated herein by reference. The present application generally relates to pressure control valves. It is beneficial when using pumps in multiple function complex circuits to control the pressure in individual legs of the complex circuits.
Pressure responsive relief valves having unload features limit the working pressure of a system and may, through electrical or pressure sensitive devices, unload hydraulic systems to a low pressure value in one mode of operation.
The operation of a pilot operated relief valve with unload characteristics having a pilot stage relief is well-known. A pilot stage relief is used to unbalance a spring loaded hydrostat which is an equal pressure sensitive device well-known to those skilled in the art. This overcomes problems which exist in direct acting relief valves. This well-known arrangement uses a fixed spring in conjunction with the hydrostat which means that the low unload value of the control can only be equal to the force exerted on the hydrostat plus the fixed spring tension during the unload condition. The spring tension must be such that it prevents instability in the hydrostat during normal relief action and unload action which generally requires a high spring tension obtainable from a heavy gauge spring. The fixed hydrostat spring is not adjustable and the only adjustment of a pilot operated relief valve is the pilot section spring. The low unload pressure in these known relief valves varies with pressure dependent on hydrostat stability, generally from about 80 psi to about 125 psi unload pressure. The pressure response in reference to the pilot section is fixed by the fixed tension hydrostat spring and, therefore, is non-adjustable. The invention described in application Ser. No. 08/121,275, now U.S. Pat. No. 5,368,061, allows the low unload pressure to be set on a separate spring other than the adjustable hydrostat spring, causing the two controls to work independently with a single load sense signal. The combination control allows a low unload of near atmosphere pressure instead of the aforementioned 80 psi to 125 psi, thereby lowering the unload horsepower draw by 95%. The combined control offers fast response and large volume flows while creating small pressure drops in reference to prior art poppet designs, and provides finely metered flow control in reference to the spool type hydrostat. This combination can be used in a variety of configurations, such as a unique pressure relief with a variable pressure cutoff or ramp; a low unload relief operating at pressures near atmosphere; a pressure relief with a variable pressure cutoff or ramp and a low unload relief operating at pressures near atmosphere; an adjustable bypass style compensator; an adjustable bypass style compensator with adjustable relief to bypass; an adjustable bypass style compensator with adjustable relief to bypass and low unload at near atmosphere pressure.